The present invention relates to a capacitor having terminals formed by ion plating and a method for making the same.
The essential components of capacitors which store electric energy are dielectric, conductors and leads. In general the so-called film capacitors may be divided into roll and stacked types. The roll type film capacitors are fabricated by depositing metal over one major surface of film made of polyester or the like, cutting the metal coated film and forming the film strips into a roll. The stacked type film capacitors are fabricated by laminating the metal coated films and cut the lamination into a desired shape. In both the roll and stacked type film capacitors terminals are formed on opposed end faces for connection with lead wires. In addition, the terminals serve to maintain the integrity of the rolled or laminated films (to be referred to as "a capacitor element" in this specification) in the original shape and to interconnect electrically between the metal coatings on the films in case of the stacked type. So far the terminals have been generally formed by metal spraying in which molten metal is atomized by compressed air and molten droplets of metal are thrown with a continuous stream of compressed air against the end face of a capacitor element. More particularly, Zn or zinc particles are sprayed over the end face of a capacitor element at an angle from one direction and then from the opposite direction. Thereafter two layers of Sn are formed in like manner. The coated terminals have a thickness of 0.5 mm.
In metal spraying, the finest particle size ranges between 5 and 50 microns so that when the surface to be coated is smooth, a poor bond results. Further problems of the metal spraying techniques are that working conditions are rather harmful to the health of workers and that the yield is poor.
In the meantime, there has been proposed to reduce the thickness of films in order to fabricate film capacitors which are compact in size, light in weight yet highly reliable and dependable in operation and inexpensive to manufacture.
The static capacitance C is given by EQU C = ES/t
where
E = dielectric constant, PA1 S = area of opposed electrodes, and PA1 t = thickness of dielectric.
Therefore, the volume of a capacitor may be decreased by reducing the thickness of film, and the area of film may be decreased by the reduction of the area of opposed electrodes required for obtaining the same static capacitance. As a consequence, the capacitors can be made more compact in size and light in weight by the reduction in thickness of film. For instance, the volume of a capacitor made with film 3.5 microns in thickness is about one eighth as small as the volume of a capacitor made with film 10 microns in thickness and having the same capacitance.
However, some problems arise when the capacitors formed with film 3.5 microns in thickness are coated by the conventional metal spraying methods for forming block terminals or electrodes at the ends of the capacitors. In case of the rolled type film capacitors, the deviation in alignment of film edges is generally less than 0.5 mm so that the bond between the block terminal and the end faces of a capacitor is weak and the contact resistance is high. As a result, power loss increases and accordingly the factor tan .delta. representative of the power loss increases. The reason why the bond between the terminals and the ends of the capacitor element is weak when the deviation in alignment of film edge is less than 0.5 mm is that in metal spraying molten droplets of metal are 5 to 50 microns in size so that they cannot enter the space between the adjacent films with the resulting decrease in mechanical bond and increase in contact resistance. However, in practice, the film edges are bent more or less because the metal particles force between them, but with the deviation in alignment of film edges of less than 0.5 mm, the film edges are not satisfactorily bent to ensure the desired bond and a low resistance.
The weak bond between the smooth ends of a capacitor element and the block terminals proves the considerable dependence of bonded strength upon the misalignment of film edges which offers teeth to which the terminals cling and upon the misregistration of films. However, it is not preferable to increase the misalignment of film edges because the step for forming capacitor elements becomes complex, the variation in dimensional accuracies increases, and the yield is poor.